Turn Backend into a sensible classoid.

Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.

While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.

unix/uxpterm.c

@@ -13,7 +13,7 @@ const int new_session = 0, saved_sessions = 0;   /* or these */
 const int dup_check_launchable = 0; /* no need to check host name in conf */
 const int use_pty_argv = TRUE;
 
-Backend *select_backend(Conf *conf)
+const struct Backend_vtable *select_backend(Conf *conf)
 {
     return &pty_backend;
 }